[racket] Typed Racket Performance Optimization

The Typed Racket Performance button in DrRacket informs me that (log v)
returns a Complex type, despite all its arguments being reals and that if I
do not want or expect a complex number as results, I mam restrict the type
of the arguments.  How does one do that?

#lang typed/racket/base

(: real-log (Float -> Float))
 (define (real-log v)
   (assert (log v) flonum?))
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